THE FLOW OF BLOOD IN THE VEINS 1051 



to the accumulation of blood in the auricles during the ventricular 

 systole. The curve c is often spoken of as the carotid elevation, and 

 has been ascribed by Mackenzie to direct propagation to the jugular 

 vein from the underlying carotid artery. He has come to this con- 

 clusion because he has not found it in tracings of the liver pulse in 

 cases of incompetent tricuspid valves. There is no doubt, however, 

 that the elevation can be seen on tracings from the inferior vena cava. 

 The explanation of its absence from liver tracings is probably to be 

 ascribed to the fact that the great mass of the liver substance is unable 

 to transmit the very rapid oscillation of pressure due to the closure of 

 the auriculo- ventricular valves. These venous pulsations are much 



Rod. art. 



JLJLJUJkJUlJlJLJU 



FIG. 421. Venous pulse-tracing from jugular vein compared with the g| 

 arterial pulse-tracing from the radial artery. 



more marked in cases of heart disease, where there is partial failure 

 of the heart pump and overfilling of the venous system, often combined 

 with incompetency of the auriculo-ventricular valves. 



Besides the favourable influences exercised on the circulation 

 through the veins by the aspiration of the thorax and the momentary 

 aspiration of the heart-beat itself, a considerable part is played in 

 the venous circulation by the contraction of the muscles of the body 

 as well as by the passive movements of different parts. The adjuvant 

 effect of passive or active movement on the circulation through the 

 veins is rendered possible by the existence in these vessels of valves, 

 which are semilunar folds of the intima projecting into their lumen, 

 and so arranged that they allow the passage of blood only towards the 

 heart. Two such valves are as a rule situated opposite to each other. 

 Every movement of a limb, active or passive, causes an external 

 pressure on the veins, and therefore empties them towards the heart. 

 Thus, in walking, each time the thigh is moved backwards the femoral 

 vein becomes empty and collapses, and fills again as soon as the leg 

 is brought forward to its former position or is flexed in front of the 

 body. When muscular movements become general, as in walking or 

 running, the active compression of the veins thus brought about 

 plays an important part in hurrying the blood into the right heart, 



